This invention relates generally to integrated electronic circuits, and more specifically to line drivers for data transmission.
A 10/125 Mbaud fast Ethernet transceiver requires a line driver that can accommodate the line code for both 10 Mbaud and 125 Mbaud data rates. In addition, the line driver must meet stringent standard specifications. Also, with today""s applications demanding smaller component size and smaller power supplies, it is desirous to implement the line driver at low voltages, within a small area, and at low power. Typical solutions to this problem at 5V have either employed two or three separate drivers, and do not offer the flexibility of programmability for various standard specifications.
In 10 Mbaud mode, the IEEE standard (Carrier Sense Multiple Access With Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, ISO/IEC 8802-3, ANSI/IEEE std. 802.3, Fourth edition Jul. 8, 1993) requires the transmitter to source a filtered Manchester code. The filtered output signal is a combination of a 5 MHz and a 10 MHz sinusoid of 5Vpp amplitude level. In addition, the standard specification on linearity requires that the transmitter must maintain greater than 27 dB harmonic distortion suppression. The allowed amplitude variation is xc2x110% about the nominal level.
In 125 Mbaud mode, two sub-modes must be supported; a three level line code, MLT3 and a two level NRZI line code. The IEEE standard (Fibre Distributed Data Interface (FDDI)xe2x80x94Token Ring Twisted Pair Physical Layer Meduim Dependent (TP-PMD), ANSI X3.263-1995, September 1995) requires that the MLT3 line code have a rise and fall-time between 3 ns and 5 ns with symmetry of 0.5 ns and peak-to-peak jitter less than 1.4 ns. The maximum allowed skew between a rise transition and a fall transition (also known as duty cycle distortion), when measured at                     V        ⁢                  xe2x80x83                ⁢        pk            2        =          0.5      ⁢              xe2x80x83            ⁢      V        ,
must be no greater than 0.5 ns. In addition, the transmit amplitude must be 1Vpkxc2x15%.
Power dissipation is also a concern. It is desirable that a given function have a lowest possible power dissipation. However, practical limitations yield an overhead power that must be dissipated. Thus, it is desired to minimize this overhead, or alternatively, maximize power efficiency. Typical twisted pair drivers for Ethernet and fast Ethernet do not work below 3V and are large because multiple drivers are implemented (J. Everitt, J. F. Parker, P. Hurst, D. Nack and K. R. Konda, xe2x80x9cA CMOS Transceiver for 10-Mb/s and 100-Mb/s Ethernet,xe2x80x9d IEEE J. Solid-State Circuits, vol. 33, pp. 2169-2177, December; and R. H. Leonowich, O. Shoaei, and A Shoval, xe2x80x9cMethods and Apparatus for Providing Analog-FIR-Based Line-Driver with Pre-Equalization,xe2x80x9d U.S. Patent submission, March 1998).
A prior art line driver is shown in FIG. 1. In 100 Base-TX mode the IEEE specifications require xc2x11Vpk between nodes vop 105 and von 110 when transmitting digital xe2x80x9c1xe2x80x9d, and 0V when transmitting digital xe2x80x9c0xe2x80x9d. Hence, a xe2x80x9c1xe2x80x9d is achieved by closing switches S1 115 and S2 120 and opening switches S3 125 and S4 130. A xe2x80x9c0xe2x80x9d is achieved by opening all four switches. Using a 1:N transformer and a peak voltage vop105xe2x88x92von 110=1Vpk, the following current is required from the line driver for RL135=100xcexa9, assuming the most power demanding 4 bit repetitive pulse sequence [0 1 0xe2x88x921] is being transmitted:                               I          av                =                                                            20                ⁢                N                            +              0              +                              20                ⁢                N                            +              0                        4                    =                      10            ⁢            NmA                                              (        1        )            
The total power dissipation is therefore 10 NVdd mW. The voltage between the nodes vpp 140 and vpn 145 is thus             V      pk        N    .
The current the device must source at its output pins is therefore increased by a factor N while the voltage is reduced by the same factor relative to the current and voltage seen at the load, RL 135. Having N greater than 1 increases the current the driver must source, hence increasing total supply power dissipation. This choice is not desirable if the goal is to minimize power supply dissipation. Choosing N less than 1 certainly helps reduce power dissipation, however this is not desirable in 100 Base-TX mode due to reduced transformer bandwidth performance and is impractical in 10 Base-T mode where a 2.5 N Vpk signal is required from a 3V supply. Therefore, a 1:1 transformer must be used. This ideal driver would thus dissipate a minimum of
Pav=(10 mA)Vdd=33 mWxe2x80x83xe2x80x83(2)
In 10 Base-T mode the IEEE specifications require xc2x12.5Vpk between nodes vop 105 and von 110. The transmit symbols are either a 10 MHz sinusoidal pulse (single bit) or a 5 MHz pulse (double bit). Thus, a 10 MHz pulse requires for all xe2x80x9c1xe2x80x9ds data (continuous 10 MHz sinusoid)                               I          av                =                                            2              π                        ⁢                          I              max                                =                                                    2                π                            ⁢              50              ⁢              mA                        =                          32              ⁢              mA                                                          (        3        )            
for a total power dissipation at 3.3V of
Pav10=105 mW, for all xe2x80x9c1xe2x80x9ds dataxe2x80x83xe2x80x83(4)
In 10 Base-T mode, receive equalization is not employed, hence there exists some cable length, Cmax, at which the transmitted 5Vpp 10 MHz pulse will be attenuated by the cable and will be too small to be detected by another entity on the Ethernet. Since cable attenuation is a function of frequency, the 5 MHZ pulse will not suffer as much loss as the 10 MHz pulse. As a result, the 5 MHz preamble will be detected and the 10 MHz sinusoid pulse will cause carrier loss. It is therefore desirable to shape the 5 MHz pulse such that after passing through a cable of length Cmax, both the 10 MHz pulse and the 5 MHz pulse have the same amplitude. Shaping of the 5 MHz pulse is signal pre-emphasis. The average current dissipated when transmitting a 5 MHz pulse is proportional to the amount of pre-emphasis. The more emphasis, the more power dissipation. Therefore, a design trade-off exists between power dissipation and maximum cable length that can be accommodated. FIG. 2 is a graph illustrating the relationship between power dissipation and cable length. In FIG. 2, region A 205 shows the over-emphasized region where more power is dispensed (more emphasis), and does not imply improved cable performance. Region A 205, is an undesirable region of operation because carrier loss will occur. Region B 210 is the proper region of operation where less pre-emphasis reduces the received 5 MHz amplitude relative to the 10 MHz pulse and hence, for error free performance, the cable length must be reduced as emphasis is reduced to reduce power dissipation. Region C 215 occurs where the less pre-emphasis makes the 5 MHz pulse width too narrow to represent a double bit at any cable length. The average current dissipated in 10 Base-T mode is therefore from (3)                               I          av                =                              50            ⁡                          [                                                                    2                    π                                    ⁢                  ρ                                +                                                      (                                          1                      -                      ρ                                        )                                    ⁢                  κ                                            ]                                ⁢          mA                                    (        5        )            
where xcfx81 represents the percentage of 10 MHz pulses over time and xcexa represents the scale factor for the current, Imax, as function of pre-emphasis. Recall from (3) that for 2.5Vpk on the 100xcexa9 load, we require Imax=50 mA to generate a 5 MHz square wave pulse. For equally likely 10 MHz and 5 MHz pulses, xcfx81=0.5, and with xcexa=0.8 (about 50% emphasis) and at 3.3V supplies we obtain a minimal power dissipation of
Pav=Iav=Vdd=119 mW, for random dataxe2x80x83xe2x80x83(6)
This condition occurs when Cmax=140 m for CAT3 cable for which the 5 MHz and 10 MHz received amplitudes are similar. This condition is depicted by point O 220 in FIG. 2. From (5) we note that if the 5 MHz shaping were not employed, Pav=135 mW, hence pre-emphasis provides a 16 mW savings when xcexa=0.8.
The present invention provides a single integrated programmable transmitter circuit, for Ethernet as well as Fast Ethernet applications including a line driver portion, a control portion, and a FIR filter portion.
The line driver accommodates binary encoded data and provides output data encoded in one of three selectable formats. These formats include Manchester encoding, MLT3, or NRZI. The line driver receives Manchester encoded data at a data rate of 10 Mbaud. MLT3 and NRZI data are encoded by the line driver by the control logic, and output at a data rate of 125 Mbaud.
In the 10 Mbaud mode, harmonic distortion is tunable, allowing a trade-off between power dissipation and performance. In the 125 Mbaud mode the transmitted data duty cycle, rise-time slew rate, fall-time slew rate, and rise-time and fall-time asymmetry are programmable, allowing a trade off between power dissipation and performance. These transmitted data parameters are adjustable. Proper selection of the adjustable parameters enables the transmitter circuit to meet specific performance requirements, such as those described in IEEE standards. Amplitude control is employed in all modes in an effort to maintain constant maximum transmit amplitude.